This invention relates generally to energy storage devices, e.g., rechargeable batteries, and especially to such devices suitable for medical implantation in a patient""s body. More particularly, this invention relates to a method and apparatus for limiting the amplitude of temperature spikes produced by such energy storage devices as a result of a malfunction such as an electric short.
Over the past several years, various medical devices have been developed which are designed to be implanted in a patient""s body. Such devices are useful, for example, for stimulating internal body tissue (e.g., muscle, nerve, etc.) for a wide variety of medical applications. Many of these devices include a battery which can be recharged from an external power source via either a hard wire connection or wirelessly, e.g., via an alternating magnetic field. Although various battery technologies have been used, the lithium ion battery has evolved to generally be the battery of choice for implantable medical devices.
Under certain malfunction conditions, such as an internal or external electric short, a rechargeable lithium ion battery can produce a temperature excursion of 120xc2x0 C. or more. A temperature of this amplitude can cause significant damage to body tissue.
The present invention is directed to a method and apparatus for containing heat generated by an energy storage device (hereinafter generically referred to as xe2x80x9cbatteryxe2x80x9d) to reduce the amplitude of a temperature excursion in order to enhance safety in temperature critical applications, such as in implantable medical devices.
In accordance with the invention, a heat absorber is closely thermally coupled to the battery. The heat absorber is comprised of high heat capacity heat absorbing (HA) material which allows, the rapid transference of considerable heat energy from the battery to the absorber, thereby reducing the temperature excursion and the risk of body tissue damage.
In accordance with a preferred embodiment, the HA material is selected to exhibit an endothermic phase change at a temperature T1 within a range of about 50xc2x0 C. to 80xc2x0 C. Temperatures in excess of this range can be readily produced by an electrically shorted battery. A preferred HA material in accordance with the invention includes paraffin and is selected to have a melting point of about 75xc2x0 C.
In accordance with an exemplary application, the battery is comprised of a case formed of a conductive biocompatible material such as titanium or stainless steel. The case is mounted in a medical device housing also formed of a conductive biocompatible material such as titanium or stainless steel. The case is preferably mounted such that its wall surface is spaced from the housing wall surface to minimize heat transference therebetween. A heat absorber in accordance with the present invention is preferably mounted between the case wall and housing wall to absorb heat energy and reduce the rate of heat transference to the housing wall.
The heat absorber preferably comprises a mass of meltable material, e.g., paraffin, deposited into a fibrous containment mat preferably formed of dielectric fibers, e.g., kevlar or fiberglass. The heat absorber can be fabricated by depositing melted HA material onto the mat and then allowing it to solidify, e.g., at room temperature. The heat absorber can then be attached to the battery case prior to mounting the case in the medical device housing.
A heat absorber in accordance with the invention can be provided in multiple configurations. As already mentioned, the heat absorber can be physically configured to engage the case wall outer surface of a standard battery. Alternatively, a battery can be especially configured to include the HA material within the battery case. For example, the HA material can be formed as one or more strips (or plates) such that they can be integrated in a stack of positive electrode, negative electrode, and separator strips (or plates) forming an electrode assembly roll (or stack). Alternatively and/or additionally, the HA material can be configured to extend around the electrode assembly within the battery case.